Studies on Range Expansion, Predation Pressure

Species, Phylogeny and Evolution 1, 3 (30.9.2008): 129-140.

Studies on range expansion, predation pressure and insolation in Albinaria on the island of Dia (Greece), focused on a recently introduced species (Gastropoda: Clausiliidae)

Colette S. Mesher1 and Francisco W. Welter-Schultes2

1 C. S. Mesher, Marine Biology and Ecology Research Centre (MBERC), University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom 2 F. W. Welter-Schultes, Zoologisches Institut der Universität, Berliner Str. 28, 37073 Göttingen, Germany, [email protected]

Abstract

We examined the morphological properties of the introduced land snail Albinaria teres (Pulmonata: Clausiliidae) on the island of Día, north of Crete, Greece. Shells of four Albinaria species were sampled and measured from 68 localities evenly distributed over the island area. We tested possible correlations of shell parameters with insolation and predation pressure effected by Drilus beetles. We could not find any correlation of shell size, shape, number of whorls or teleoconch ribs of any species with insolation and predation pressure. The endemic A. torticollis, A. retusa and A. jaeckeli were more heavily predated by Drilus than the invader A. teres, which was morphologically less variable. The spatial distribution of shell size in A. teres exhibited a concentric pattern, with the largest snails being found at the periphery of the population's range, they decreased in size towards one site at the northwestern coast of the island, interpreted as the locality of introduction. Our observations suggest that A. teres is expanding its range and that during the process of range expansion, larger individuals have had a selective advantage (migration ability hypothesis). To determine the range expansion velocity C, we could not apply the C = (2 r D)0.5 formula, since we neither had experimental data to estimate the intrinsic growth rate, r, nor a previously determined spread rate to deduce the diffusion coefficient, D. We developed a model based on known data about annual displacements of individuals and the distribution of age classes in a natural population. Our simulation produced a mean rate of spread of 0.90-1.35 m yr-1, corresponding well with known land snail range expansion velocities. The mean distance between full population density and the first invaders of the new territory was 7.3 ± 1.7 m. If these values corresponded with reality, then the species was introduced at about 300 BC in the Greek period of Crete.

Résumé

Nous avons examiné les propriétés morphologiques du gastéropode terrestre Albinaria teres (Pulmonata: Clausiliidae) introduit à l’île de Día, au nord de la Crète, Grèce. Des coquilles de quatre espèces d’Albinaria ont été ramassées et mesurées de 68 stations distribuées sur toute l’île. Nous avons essayé de trouver des correlations des paramètres de la coquille avec l’insolation ainsi qu’avec la pression de prédation effectuées par des coléoptères du genre Drilus. Nous n’avons point trouvé une telle correlation ni avec la grandeur, la forme, le nombre de spires et les costilles de la coquille, dans aucune espèce. Les espèces endémiques A. torticollis, A. retusa et A. jaeckeli souffrirent plus fortement sous la prédation de Drilus que l’espèce invasive A. teres, qui était morphologiquement moins variable. Nous avons observé un cadre concentrique dans la distribution spatiale de la grandeur de la coquille d’A. teres, dont les coquilles les plus grandes ont été trouvés à la périphérie de l’aréal de distribution, diminuisant en grandeur vers une certaine localité au nord-ouest près de la côte, probablement l’endroit d’introduction, de sorte que les grandes individus ont eu un avantage sélective. Pour déterminer la vélocité d’expansion d’aréal de distribution C, nous n’avons pas pu utiliser la formule C = (2 r D)0.5, car nous n’avions ni des dates expérimentales pour estimer la vitesse interne de grandir r, ni une vitesse d’expansion déterminé avant pour déduir le coefﬁcient de diffusion D. Nous avons développé un modèle basé sur des dates connus des movements annuaires des individus et la distribution des classes d’age dans une population naturelle. Notre simulation a produit une vitesse moyenne d’extention d’aréal de distribution de 0.90-1.35 m yr-1, correspondant bien aves des vélocités connus pour des gastéropodes terrestres. La distance moyenne entre une densité de population complète et les premiers invaseurs du nouveau territoire était à 7.3 ± 1.7 m. Si ces valeurs correspondent avec la réalité, l’espèce doit avoir être introduite environs vers 300 avant J.-C. dans l’époque grècque de la Crète. 129 Fig 1. Aspects of the landscape of Día. A. Boulder in square 53, at which 11 adult specimens of A. teres were found. Scenery seen from west; the approximate eastern range limit of the species is visible in the background (dotted line). B. Landscape of southern Día seen from the highest elevation at 268 m. Apart from local differences in the exposure, the island consists of a very uniform environment. Crete can be seen in the background.

Introduction more size-dependent than of flying animals (Peters 1983). Predicting the rate of spread in animals bears two major Range expansion and invasions of new territories have problems. (i) For flying animals long-distance dispersal received considerable attention over the past decades is easier than for wingless animals. This increases the (Elton 1958, Lawton & Brown 1986, Baur & Bengtsson probability in flying animals that new territories will be 1987, Lubina & Levin 1988, Hengeveld 1989, Okubo et colonized not only by the populations at the periphery of al. 1989, Van den Bosch et al. 1990, 1992, Cushmann et the species' range. This will distort calculations based on al. 1993, Grosholz 1996, Lensink 1997). This paper is a pure neighborhood diffusion model (Hengeveld 1989). focused on whether larger land snails are more successful (ii) Some animal groups are sexual and have complicated invaders than smaller sized ones of the same species. Such social structures, for example terrestrial mammals. Also a study has never been undertaken in land snails. We have here, range growth cannot be described as a pure diffu- some data from insect studies, the conclusions of which sion-like mechanism, as for example in the case of Brow- can certainly not be directly applied for land snails, but nian motion of particles (Caughley 1970, Clarke 1971, we think they are interesting and worth to be cited in the Okubo et al. 1989, Van den Bosch et al. 1992). introduction. Being hermaphroditic animals without complicated A correlation between insect body size and success social structures, land snails are potentially more con- in migration or range expansion has been suggested in venient to study the nature of these processes. Their low several studies (Roff 1977, Harrison 1980, Cushmann et powers of dispersal additionally predestine land snails for al. 1993, Gutiérrez & Menéndez 1997). Cushmann et al. biogeographical studies. Land snails are good at coloni- (1993) introduced the expression "migration ability hypo- zing new territories either by natural means (Peake 1969, thesis" for this idea, but rejected this option to explain Vagvolgyi 1975, Valovirta 1979, 1984, Baur & Bengtsson the presence of larger-sized ants in more recently coloni- 1987) or by artificial dispersal by humans (Gittenberger zed regions of central and northern Europe. Size-depen- & Ripken 1987, Johnson 1988). In many regions of the dent dispersal ability of Drosophila flies was suggested eastern Mediterranean, the proportion of introduced snail by Roff (1977) who concluded that uncolonized habitats species exceeds 30 % (Mylonas 1984, Welter-Schultes should be colonized by larger individuals from the main 1998b, Welter-Schultes & Williams 1999). Snails were population. frequently carried accidentally on ships in the antiquity The velocity of range expansion has also been the sub- (Welter-Schultes 2008). ject of intensive research in both theoretical and experimental approaches (Lawton & Brown 1986, Lubina & Levin 1988, Okubo et al. 1989, Van den Bosch et al. 1990, The study object: Albinaria teres 1992, Matis et al. 1995, Grosholz 1996). As a general rule, larger animals can cover larger distances at higher velo- Albinaria is known for its fascinating variety of more cities, with velocities of strictly terrestrial animals being than 120 species and innumerable shell forms in Alba-

130 Fig 2. The island of Día north of Crete with 20 m contour lines, sampling grid and sampling sites. Akra Síderos in eastern Crete is the assumed origin of the artificially introduced A. teres population of Día. nia, Greece, Turkey, Cyprus and Lebanon (Böttger 1878, 1). The whole island is covered by shrubs of less than 1 Nordsieck 1977, 1999, Welter-Schultes 2000b). The high m in height, growing in a karstic terrain of pre-Neogene species number and the morphological variation does not (middle Jurassic to Eocene) limestone outcrops (IGME appear to be associated with a similar amount of ecolo- 1996). Día is protected by the Dasikí Ipiresía (Forestry gical differentiation, an observation that led Gittenberger Supervision) of the Cretan government and is mainly used (1991) to suspect non-adaptive radiation in the evolution to conserve one of the last populations of the Cretan wild of the genus. goat (Capra aegagrus cretica). The only environmental In Crete with its more than 20 species, this radiation is variable between sites seems to be the inclination of the believed to have occurred prior to the Tortonian (Douris hill slopes combined with their exposure (Fig. 1). The et al. 1998, Welter-Schultes 2000a). Some Albinaria highest elevation is at 268 m above sea level. populations have been artificially dispersed, for example Día is inhabited by four Albinaria species, three of from western Crete to the island of Lésvos (Nordsieck which were among the first land snails described from 1977, Bank 1988) or within Crete from one site to Greece (Olivier 1801, Wiese 1989, Schultes & Wiese another, over trajectories of 3 to 20 km (Welter-Schultes 1990, Welter-Schultes 1992). A. torticollis (Olivier, 1998b). Albinaria species exhibit a high degree of spatial 1801), A. retusa (Olivier, 1801) and A. jaeckeli Wiese, variation, so it is often possible to determine the origin of 1989 are endemic species of this small island. Employ- an introduced population. Some Albinaria characters such ing a morphological approach we will compare A. teres as shell size, shape or whorl numbers are not associated with the endemic species of the island. In this sense we with geological substrata, vegetation types, altitude and will test possible differences between the spatial variation predation pressure (Welter-Schultes 2000b, c). Humidity of the morphological characters among the species, and has not been tested. Insolation will be tested here. examine eventual correlations with environmental factors Our study is focused on the introduced population of A. (degree of insolation and predation pressure). Finally, to teres (Olivier, 1801) on the island of Día, 10 km north of learn more about the history of A. teres, we will try to central Crete. A. teres occupies a compact range of 1500 develop a model to assess the range expansion velocity. km2 in southern central and eastern Crete, at a distance of at least 70 km from Día (Welter-Schultes 1998a, b). The origin of the A. teres population of Día must have been Materials and methods the easternmost margin of Crete (Akra Síderos), 100 km distant from Día (Welter-Schultes 1998b). The Greek island of Día was divided into 67 squares The 12 km2 sized island of Día represents a very uni- based on a 500 x 500 m UTM grid (Fig. 2). Empty Albi- form environment, in the sense that it hosts neither differ- naria shells were collected in 1990 by J. Fischer and ent vegetation types nor different types of substrates (Fig. myself from each square in which they occurred (A. teres

131 Table 1. Population growth matrix for one m2 and one year (we arbitrarily selected 1995). In a two-dimensional calculation program (quattro pro, excel), many matrices of this form would cover the area, with left-hand and right-hand neighbor matrices corresponding to the neighboring square meters west and east, and upper and lower neighbor matrices corresponding to the previous and coming years, respectively. Column F: total proportion of migrants 0.70, of which 50 % have no inﬂuence because they migrate to northern and southern directions. Coordinates without year refer to 1995.

47 samples, A. torticollis 42, A. retusa 14 and A. jaeck- hine at western slopes might be subjectively more inten- eli 5 samples). The sites of collection did not necessarily sive than at eastern slopes due to higher temperatures in correspond with the center of the square (Fig. 2). Sampling the afternoon. area was in most cases less than 10 x 10 m, rarely up to We tested three species, A. torticollis, A. retusa and A. 30 x 30 m, an area within which Albinaria does not show teres. For shell size we took the diameter because other- major morphological differentiation outside hybrid zones wise the decollated species A. torticollis and A. retusa (Schilthuizen 1995). could not be compared with A. teres. Since relative shell The material is stored in the malacological museum height has a different meaning in decollated shells, only Haus der Natur, Cismar (Germany). In a maximum of A. teres was tested for relative shell height, and for the 12 randomly selected shells per sample, shell height (H), same reasons also for the relative number of whorls Wh/ln shell diameter (D), number of whorls (Wh) and number H. of ribs on the penultimate whorl (RP) were determined. H The obtained mean values for each station were plotted and D were measured with a micrometer (accuracy 0.005 against the insolation angle. Since the numbers of speci- mm, determined error ± 0.015 mm). The determined error mens was different in each sample, a correct application for RP was ± 3.0 %. For whorls and more information on of statistical tests was not possible. methods see Welter-Schultes (2000b).

Predation pressure Insolation The most significant predator of Albinaria is a beetle of the Insolation is correlated with the direction of the exposure genus Drilus (Coleoptera: Drilidae) that attacks the snail of the locality combined with the degree of slope (incli- via a self-made perforation in the shell (Schilthuizen et al. nation). The island of Día is located at 35°27‘ N latitude. 1994, Örstan 1999). To determine the degree of predation, Highest sunshine intensity is expected at south-directed in each sample of more than 10 specimens the proportion 35° slopes, gradually lower degrees of insolation are of shells with oval boreholes originated by drilid beetle expected at lower angles. We calculated the angle of inso- attacks were examined. Samples with less than 10 dead lation for 21 March/23 September and 12.00 h which was collected specimens were not considered. Juvenile speci- 90° at S-directed 35° slopes, 0° at N-directed 65° slopes, mens without clausilium apparatus were separated and 55° in flat areas and for example 17.5° for perpendicular not considered in the analyses. Only successful attacks SW- or SE-directed (90°) slopes. In our calculations we were regarded as boreholes, signs of unsuccessful bore neglected that the sea reflects the sun, potentially cau- attempts were neglected. Boreholes in the aperture were sing slightly higher degrees of insolation at the southern, not considered as successful attacks. The shells were not western and eastern coasts. We also neglected that suns- opened to verify eventual perforations of the clausilium.

132 Drilus could theoretically have used more frequently the old ones, we would expect that 1.15 snails hatch in a clausilium as the entrance in thick-shelled snails, but from population of 3.63 individuals. This represents a propor- Crete we know that Drilus predation pressure probably tion of 0.65 hatched snails per adult in the population. makes no difference between thick and thin shelled Albi- However, this is too low, as it is unlikely that more than naria species. We have more data on Drilus predation in 80 % of the hatched snails survive the ﬁrst year. A better Crete and we know that we need to publish them in the approach seems to be 35 %, which means 2.23 hatchlings near future. per adult per year. To approximate these patterns we arbitrarily employed quadratic functions, although these pro- vide realistic results only for the ﬁrst 20 years. For unin- Velocity of range expansion habited terrain, we slowly increased the survival rate to 50 %, applying the function for the survival rate of the

To calculate the speed of range expansion we developed a hatchlings sh matrix in Quattro Pro, a commonly available calculation s = 0.3 × pd2 - 0.75 × pd + 0.8 program (Table 1). This matrix was fed with data obtained h from Albinaria corrugata (Bruguière, 1792) population We also increased the fertility in areas of lower population size and dispersal rates (Schilthuizen & Lombaerts 1994), density, assuming that the snails are better fed and can from which growth rates and dispersal could be inferred. produce more eggs. We assumed the number of hatchlings per adult to be 5.0 at a population density of zero, using a Growth rates. Schilthuizen & Lombaerts (1994) found a fertility equation mean of 3.63 individuals per m2 in habitats that are more h = 10 × pd2 - 22.767 × pd + 15 or less similar to the situation in Día. Of those, 51 % were juveniles, the rest were adults. The death rate was not where h = the number of hatchlings per adult. determined, but Schilthuizen & Lombaerts (1994) sus- pected that adult mortality was so high that the majority Dispersal. The mean annual displacement of one Albi- of breeding individuals are replaced by a new genera- naria individual is 1.45 ± 0.86 m yr-1, without substantial tion each year. Albinaria needs at least 2 years to reach differences between juveniles and adults (Schilthuizen & maturity, so Schilthuizen & Lombaerts (1994) assumed a Lombaerts 1994). This might be because of a difference in generation time of 3 years. We considered all 1-year and the behaviour of juveniles, which in contrast to adults are 2-year old animals as juveniles and 3-year and older ani- not occupied with reproductive activities and thus, might mals as adults. disperse more intensively, neutralizing the effect of the We took a constant mortality rate for juveniles, let the smaller body size of juveniles. death rate increase by the factor 2 in 5-year old and 6-year The dispersal function can be written in the form of a old adults, and neglected all adults older than 6 years. Gaussian normal distribution The number of individuals in each age class was deter- f(x) = c × (1/(0.86 × (√(2π))) × e exp(-(x-1.45)2/2 × (0.86)2)) mined as where x is the distance travelled by the snails, and c is a n × ax 0 constant depending on the population size (c = 0.04109 where a represents the initial survival rate (for example, at 3.63 individuals per m2). However, this function would a = 0.4 if 40 % of the juvenile animals survive in 1 year), produce very low proportion of snails travelling more than x is the age (in years), n0 = number of individuals at age 3 m (only 0.6 %). The empirically determined fraction of zero. snails having moved a distance of more than 3 m in 1991-

The factor a0 = 0.864 will produce the observed pat- 1992 was 5.5 % (Schilthuizen & Lombaerts 1994). There terns (51 % juveniles and 49 % adults, where 42 % of the might be biological reasons rather than statistical ones adults are 3-year old). behind this pattern. In our calculations we used the basic The survival rate may be increased at lower popula- data provided by Schilthuizen & Lombaerts (1994), with tion densities at the periphery of an expanding population, 57 % of the snails in the 1 m annual displacement section, because the food supply is higher in the new terrain. So for 8 % in the 2 m section, 3 % in the 3 m section, and 2 % juveniles and adults we corrected the survival rate assu- in the 4 m section (total proportion of migrants 0.70). The ming half mortality at a population density of zero, and a number of immigrants from one direction (here: immi- linear increase in mortality towards maximum population grants from east, IE) in each age class is density: × × × × IE = 0.25 (0.57 np+1 m + 0.08 np+2 m + 0.03 np+3 m × s = -0.068 pd + 0.932 + 0.02 np+4 m) where s is the survival rate and pd the relative popula- where np is the total number of individuals in the same tion density (total number of snails in the previous year age class of the previous year (for immigrants from west divided by 3.63). To avoid circular conclusions in the (IW) it would be np-1 m and so on). calculations we had to take the population density of the previous year. Rate of spread. In larger populations such as A. teres in It is not known how many eggs are laid and how many Día, spread is a one-directional movement (here: from snails hatch under normal circumstances. Assuming the west to east). Given that the animals move randomly over same mortality for 0-1-year old juveniles as for 1-2-year area, 25 % of the animals will move eastwards and 25 % 133 Fig 3. Spatial variations of shell diameter and rib numbers in A. torticollis and A. retusa. Insert pictures: Upper row, A. torticollis, H = 10.7 mm, specimen has a drilid borehole. Lower row, A. retusa; left-hand specimen, collected at Anginára in the sourthern part of the species‘ range, H = 13.2 mm; right-hand specimen, form of Kaltoúka (without distinct ribs on the upper whorls), H = 14.2 mm, specimen has a drilid borehole.

This factor should be slightly larger for the east than for the west, because individuals will stay for longer time in less populated terrain due to higher food supply. We did not consider this effect.

Results

Spatial variation of shell parameters

A. retusa and A. torticollis exhibited considerable morphological variation across the study area (Fig. 3). A. torticollis had large values in most regions, with a few intermediate and small ones in the northwest, extreme west and central south (Fig. 3). There was a general west-east gradient from smaller shells in the west to larger shells in the east. The number of ribs was high in the east and µ Fig 4. Relative standard deviations (100 sd/ ) of all available southeast, but the rest of the study area contained small sample means from Día taken together for each species (A. teres 48 samples, A. torticollis 47, A. retusa 14), reflecting the degree values. of spatial variation over the island. To calculate sd and µ, the lots Shell size in A. retusa was large in the northeast but were not weighted for sample size (they had different numbers small at the northern and eastern margins (Fig. 3). The of specimens). This was as arbitrary as the choice of the loca- values for the number of ribs were more scattered, with tions of the corresponding stations, so statistically testing the high and intermediate values in the north, intermediate significance of the difference between A. teres and the endemic and low values in the east and all types of values in the species would not improve the result. center of the species' range. In A. teres we observed some remarkable deviations from usual plots. When we compared shell diameter and rib number of A. teres with those of A. retusa and A. tor- westwards. The mean distance travelled eastwards by a ticollis, the degree of spatial variation in A. teres was sig- population in one year is nificantly lower (Fig. 4). 0.25 × 1.45 m × cos (22.5 °) = 0.25 × 1.34 m = 0.335 m Shell size (HDD) in A. teres increased gradually from

134 Fig 5. Spatial variations of shell size, shape, relative whorl numbers and rib numbers in A. teres. Insert pictures show A. teres from Día, H = 21.0 mm. Modiﬁed in the upper row to illustrate the effects of size and shape.

Fig 6. Spatial distribution of correlation coefficients (cc) of A. teres shell size (HDD) plotted against the distance between the indicated localities and the sampling sites. The best correlation was obtained at the eastern margin of Plataniás bay, at the northwestern coast. All shell size values increased concentrically around this site. 20 m altitude contours. Sites 8 and 53 are indicated. Dashed line, eastern range limit of A. teres as observed in 1990. northwest to southeast (Fig. 5). There was no linear gra- ent candidate sites and sampling sites (Fig. 6). One site at dient as was observed in A. torticollis, but the values the northern coast of the western peninsula (Plataniás bay) increased concentrically towards all directions, rooted at produced the best correlation (c.c. 0.574). Most neighbor- one particular site in the western part of the island. We ing sites at a distance of 500 m had slightly lower values tested the origin of the circle by comparing correlation (c.c. 0.54-0.55). The correlation coefficients correspond- coefficients of shell size and the distance between differ- ing to the sites at larger distances decreased gradually. We

135 Fig 7. Distribution of shell size (HDD) in two samples (8 and 53, locations shown in the insert maps) of A. teres. found significant differences in the mean values between representative eastern and western populations, but no dif- Fig 8. Spatial distribution of the proportion of shells attacked ference in the variance. The extreme populations showed by drilid beetles. a low degree of overlap (Fig. 7). The shells of station 8, very close to the assumed locality of introduction, were smaller than those from the 3 km distant eastern station significantly lower for A. teres (mean proportion in these 53 at the margin of the populations‘s range (t-test, P = samples 17.4 %) than for the other three species (53.1 %) 0.0000002, null hypothesis: homogeneity of mean). There (two-tailed t-test, P = 0.0042). was no significant difference in the variance of both samples (f-test, P = 0.3549, null hypothesis: homogeneity of variance). Velocity of range expansion The relative shell height (H/D) of A. teres had mostly high values, except for a few smaller ones in the center of Our model was simple when compared with the much the study area. The relative whorl number showed aggre- more complicated patterns natural populations are con- gations of low values in the center of the island, gradually fronted with. It gave good results for the first 20 years increasing southwards, northwards and westwards. The after establishment of the first 0.001 snails in a square number of ribs had high values in the west and in the east, meter, but the populations exploded or vanished after intermediate values in the north, and low values in the 25-30 years. Exploding of populations is not unusual in central and southern parts of the island. calculations of population sizes and is often observed in this kind of models. In our case it was mainly because the approximations determining the survival rates and the Correlation with environmental factors number of hatchlings were not sensitive enough to cope with exceptionally high snail numbers, and in contrast to The plots for insolation gave no visible trends in any spe- nature the model had no regulators to reduce high popula- cies for any of the shell parameters shell size, rib densities, tion densities. Unlike our model which had to work with relative shell height and relative whorl number (Fig. 9). population densities of the previous year, nature has no All values were evenly distributed in the plots and no con- need to avoid circular conclusions as we had in our arith- centrations pointing to any concentration of values could metics and can regulate immediately. This step produced be spotted. We had angles of insolation between 0 and 90 high 2-year-oscillations after 25 years. We also did not degrees, most values were located in the 40-60 degrees delete negative numbers of individuals, which resulted section. For making visible a possible correlation between from more snails immigrating than emmigrating in some insolation and any of the shell parameters, the study pro- rare cases. Despite these shortcomings, the model provided enough values in the lower and higher insolation vided a stable simulation of an equilibrial population for sections. But these sections produced average values as the first 20 years. statistically expected for evenly distributed characters. In the equilibrated system the distance between the area A. teres suffered less predation (mean proportion with full population density (3.63 snails m-2) and the area of attacked shells in all samples 17.5 %) than the other without snails (< 0.001 snails m-2) was 12.0 ± 1.3 m. If three species A. torticollis (mean proportion 39.3 %), A. we consider a 1 m square with 0.05 individuals as signifi- retusa (51.1 %) and A. jaeckeli (54.2 %) (Fig. 8). At five cantly inhabited by snails, then the distance between full sites we were able to analyse sympatric occurrences of A. population density and the first conquerors at the periph- teres with A. torticollis (2 sites), A. retusa (2 sites) and A. ery was 7.3 ± 1.7 m. jaeckeli (1 site). In all cases the predation pressure was As expected, the equilibrated system shifted eastwards

136 Fig. 9. Shell diameter (D) and rib density (ribs penultimate whorl RPm) plotted against the angle of insolation in A. torticollis, A. retusa and A. teres on Día. For the relative shell height and the relative number of whorls data could only be obtained for the non-decollated A. teres. each year. The average eastward-directed movement of Discussion the square meters that were populated by 0.2-2.5 individuals, or the velocity of range expansion, was 0.92 ± 0.10 Correlation with insolation m yr-1. Several parameters that might have enhanced the Our results showed no correlation between insolation and spread velocity could not be considered in our simula- shell size, relative height, number of whorls, number of tions, because more complicated models were less stable ribs or rib densities (Fig. 9). It seems that exposure and and resulted in earlier population explosions. Thus we insolation have no directly visible effect. neglected that the migration might be biased towards areas With insolation we have indirectly examined the effect with lower population densities: snails will prefer to stay of temperature on shell characters. The size of xerophil- in areas with higher food supply. We tried to simulate this ous snails responds better to temperature than to humid- in the model by determining that 30 % of the animals will ity (Rensch 1932). Temperature is undoubtedly the most move eastwards and 20 % westwards. In the few years important aspect of altitude (Barry 1992). Most land snails when this model remained stable, the species expanded at show a negative correlation of shell size with altitude, a higher velocity of about 1.05 m yr-1. We also neglected although the opposite trend has also been demonstrated our empiric observation that expansion is biased towards (Goodfriend 1986, Baur 1988, Madec 1989, Pazylov larger individuals in a population. 1991, Stiven & Foster 1996). In Albinaria no correlation of shell size or relative height with altitude or temperature has been found, with the exception of A. idaea (Pfeiffer, 1850) (Kemperman 1992, Welter-Schultes 2000c). This species shows a positive non-linear relation between rib density and altitude, visible at altitude differences exceeding 1000 m (Welter-Schultes 2000c). None of the three species analysed here showed any correlation

137 between rib density and temperature. locality of introduction and those that live at the extreme It seems likely to us that the differences in temperature periphery in the east. If larger animals possess an evolu- between northern exposed and southern exposed sites on tionary advantage during range expansion (Cushmann et the island of Día do not correspond to a 1000 m difference al. 1993), then we would expect the mean shell size to in altitude which would be necessary to see the effect on increase towards the periphery of the territory currently rib density. It is also possible that the correlation that was being invaded by the species. found in A. idaea has to be explained otherwise than as a Since we have good reasons to suspect that Día provi- mere effect of temperature. des a uniform environment for Albinaria snails, species are expected to expand their range concentrically from the point of introduction. This regular pattern was not obser- Correlation with predation pressure ved in the western range limit of the introduced population of Theba pisana in Rottnest island, Western Australia We found a significant difference between the degree of (Johnson 1988), possibly because of differences in the predation in A. teres as compared to the three endemic environmental structure of the island. species. This difference is not likely to have been caused by the prominence of the ribs, although this was one of the main obvious differences between A. teres and the Velocity of range expansion and history of intro- endemic species. A. teres had a smooth white and solid duction shell, where most populations of the other species had more prominently ribbed shells, particularly A. torticollis Our model provided a mean spread velocity of 0.9-1.1 and A. retusa. However, A. jaeckeli and the A. retusa form m yr-1. The analyses of the shell parameters suggest that of Kaltoúka (Welter-Schultes 2000b) had smooth shells larger individuals are faster dispersers than smaller ones but still suffered from high predation pressure. of the same species. The size of A. teres at the periphery Another obvious difference is that in contrast to A. was about one-third larger than near the assumed locality teres all three endemic species are decollate. But the high of introduction (Fig. 7), so the velocity of range expan- degree of predation cannot be explained with the shells sion in Día should be about 1.35 m yr-1. being decollate, since at many sites in Crete non-decol- This velocity is consistent with known land snail data. late species are subjected to the same degree of preda- In the slightly smaller chondrinid Chondrina arcadica cli- tion pressure as observed in the decollate species in Día enta (Westerlund, 1883) a mean annual displacement of (Welter-Schultes 2000c). In some parts of eastern Crete 0.8 m yr-1 was found (Baur 1988). The helicid Cylindrus more than 50 % of the shells of A. teres have drilid bore- obtusus (Draparnaud, 1805) with orculid habitus covers 3 holes (Welter-Schultes, unpublished data). cm per day in an activity period of 4 months (Bisenberger A possible explanation for the low degree of predation et al. 1999). Previous estimations for Albinaria were in on A. teres could be the ecological behaviour. The three the same range of the results of our model (1-2 m yr-1, endemic species are "ground dwellers" (Gittenberger Schilthuizen & Lombaerts 1994). 1991), but A. teres is often found estivating on the faces At one site of Día we found 11 adult individuals at one of the rocks, exposed to the sun. If the drilid beetles of Día boulder (Fig. 1A), at a low distance of less than 50 m from have adapted their behaviour to find their prey in crevices, the eastern range limit. This observation seems to support then we would not expect them to look for snails on the our calculated distance between full population density rock faces. A low degree of predation on A. teres is an and the first invaders of 7.3 ± 1.7 m. expected pattern for an introduced species. Given that the Our model was not sensitive enough to predict realis- population density of the other species did not decline, tic snail numbers some years after the population density there was no need for the beetles to find a new source of in a square had exceeded 5-6 individuals. Schilthuizen & prey. Lombaerts (1994) found population densities in central Crete between 2.8 and 5.2 individuals per m2, with high variances according to the habitat structure. In Crete it is Migration ability hypothesis frequently observed that groups of 50-100 animals esti- vate at the face of one limestone boulder. It is difficult to The low degree of spatial variation of shell size and rib predict reliably a natural population over many years by numbers of A. teres is in agreement with the population a simple model. Too many factors that regulate natural being the product of a recent introduction. In recently population sizes must remain unconsidered. colonized areas, the genetic and morphological diversity It was difficult to develop a model of population increase is generally reduced due to founder and bottleneck effects that fitted the population structure given by Schilthuizen (Johnson 1988, Hewitt 1996, Durka 1999). & Lombaerts (1994). To obtain a proportion of only 51 Shell size of A. teres populations increased gradually % of juveniles we had to set their survival rates at 85 %, with the lowest values being centered at one particular which might have been too high. It seems more probable point on the western peninsula (Plataniás bay). The cons- that Schilthuizen & Lombaerts (1994) overlooked many picuous limit of the eastern range of A. teres in Día stron- small juveniles. In a natural population of the clausiliid gly suggests that the species has been introduced in the Macrogastra ventricosa (Draparnaud, 1801) from Göttin- west. A. teres shows a surprisingly low overlap in shell gen, Germany, we found a proportion of 92.4 % juveniles size between populations that live close to the assumed (month of August, 2000).

138 Mainly because we did not know the time and locality Linnean Society 35: 247-259. of introduction, we could not work with the formula C Baur, B. & Bengtsson, J. 1987. Colonizing ability in land snails = √(2 r D) often employed to estimate the rate of spread on Baltic uplift archipelagos. - Journal of Biogeography 14: (Okubo et al. 1989, Van den Bosch et al. 1990, 1992, 329-341. Barry, R. G. 1992. Mountain weather and climate. 2nd edition. Grosholz 1996, Lensink 1997). We had no experimen- - London, New York: Routledge. tally determined data on population increase of Albina- Bisenberger, A., Baumgartner, G., Kleewein, D. & Sattmann, H. ria populations, so it was not possible to determine r, the 1999. Untersuchungen zur Populationsökologie von Cylin- intrinsic rate of increase. The diffusion coefficient, D, drus obtusus (Draparnaud, 1805) (Pulmonata, Helicidae). was even more difficult to determine. It should be pro- - Annalen des Naturhistorischen Museums Wien 101 B: 453- portional to the mean squared displacement per time unit 464. (Grosholz 1996). But where determined, this factor is usu- Böttger, O. 1878. Systematisches Verzeichnis der lebenden ally deduced from the empirically found range expansion Arten der Landschnecken-Gattung Clausilia Drap. mit aus- velocity, always based on populations with well-known führlicher Angabe der geographischen Verbreitung der ein- zelnen Species. - Bericht über die Thätigkeit des Offenbacher locality and time of introduction. We saw no method to Vereins für Naturkunde 17/18: 18-101. determine D independently from C, and to work with the Caughley, G. 1970. Liberation, dispersal and distribution of formula developed by Van den Bosch et al. (1990, 1992) Himalayan thar (Hemitragus jemlahicus) in New Zealand. - to trace back the unknown history of an introduced popu- New Zealand Journal of Science 13: 220-239, 1 map. lation. Clarke, C. M. H. 1971. Liberations and dispersal of red deer Our model was mainly based on empirically deter- in northern South Island districts. - New Zealand Journal of mined data on annual displacement, and combined with Forestry Science 1: 194-207, 8 maps. the known population structure provided it a suitable Cushmann, J. H., Lawton, J. H. & Manly, B. F. J. 1993. Latitu- approach towards what might have happened on Día. If dinal patterns in European ant assemblages: variation in species richness and body size. - Oecologia 95: 30-37. we take the bay of Platanías as the locality of introduc- Douris, V., Cameron, R. A. D., Rodakis, G. C. & Lecanidou, tion, then the most distant eastern range limit of A. teres R. 1998. Mitochondrial phylogeography of the land snail was at 3.2 km. This would correspond to the introduction Albinaria in Crete: long-term geological and short-term event having occurred about 2300 years ago, during the vicariance effects. - Evolution 52: 116-125. Greek period of Crete. Land snails in the eastern Mediter- Durka, W. 1999. Genetic diversity in peripheral and subcentral ranean are known to have been accidentally carried on populations of Corrigiola litoralis L. (Illecebraceae). - ships since more than 3000 years (Welter-Schultes 2008). Heredity 83: 476-484. However in these times, the northern coast of Crete was Elton, C. S. 1958. The ecology of invasions by animals and not a center of commercial trading. It became more impor- plants. - Methuen, London. Gittenberger, E. 1991. What about non-adaptive radiation? - tant after 824 AD, when the city of present-day Iraklio Biological Journal of the Linnean Society 43: 263-272. was founded, and the marginal sites of northern Crete Gittenberger, E. & Ripken, T. E. J. 1987. The genus Theba became more interesting as potential refuges for pirates. (Mollusca Gastropoda: Helicidae), systematics and Among these were the small islands, but also the strategi- distribution. - Zoologische Verhandelingen 241: 3-59. cally important distant cape of Akra Sideros from which Goodfriend, G. A. 1986. Variation in land snail shell form and our A. teres population must have come from, and which size and its causes: a review. - Systematic Zoology 35: 204- still serves as a base for the Greek navy today. The eastern 223. range limit of A. teres depicted in Fig. 1 should move east- Grosholz, E. D. 1996. Contrasting rates of spread for introduced wards, and our hypothesis could be verified after several species in terrestrial and marine systems. - Ecology 77: 1680- 1686. decades or centuries. We also predict a larger shell size for Gutiérrez, D. & Menéndez, R. 1997. Patterns of distribution, the individuals of these populations. abundance and body size of carabid beetles (Coleoptera: Caraboidea) in relation to dispersal ability. - Journal of Bio- geography 24: 903-914. Acknowledgements Harrison, R. G. 1980. Dispersal polymorphisms in insects. - Annual Reviews of Ecology and Systematics 11: 95-118. We are grateful to H. Cohen and F. Schweickert for helpful Hengeveld, R. 1989. Dynamics of biological invasions. - Chap- discussions. J. Fischer assisted in the field research on the inhos- man and Hall, London, New York. pitable island in 1990, I. Thiel in 1987. We would like to thank Hewitt, G. M. 1996. Some genetic consequences of ice ages, and the Dasikí Ipiresía of Iráklio for allowing us to visit the island their role in divergence and speciation. - Biological Journal and to collect shells of land snails in 1987 and 1990. 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